Preparation and thermoelectric properties of β-Ag 2Se

Abstract

Polycrystalline and single-crystalline specimens of β-Ag 2Se, variously doped, were prepared by direct reaction of the elements in evacuated quartz tubes. Measurements were made of electrical resistivity, Seebeck coefficient, Hall coefficient, and thermal conductivity as functions of temperature. The experimental variations of these quantities can be accounted for fairly satisfactorily in terms of a conventional conductionplus-valence band model with a small energy gap (∼0·03 eV at room temperature). The theoretical maximum value of ZT at room temperature for n-type material is about 0·65 ( Z = 2·2 × 10 −3/°K), as determined from the characteristic curve of κ/ σT versus Seebeck coefficient based on the above model. The largest measured room-temperature value of Z is 2·3 × 10 −3/°K, and specimens with both greater and smaller carrier concentrations have lower Z values, in agreement with the theory. Optimum n-type carrier concentration corresponds closely to that of stoichiometric Ag 2Se, and it has not been found possible to prepare β-Ag 2Se which is p-type above about 80°K. It is shown that the value of Z(max) T for Ag 2Se is limited primarily by the small size of the energy gap, and that any further decrease in the already small room-temperature value of the lattice thermal conductivity (∼0·002 W/cm-°C) would have little effect on Z(max) T.